Fluorescent screen



Jan. 9, 1940. F ET 2,186,393

FLUoREs'cENT ,SCREEN Fild Dec. 15, '1937 g .'/nventan:

Patentci Jan. 9, l

UITE

P 'I' l Weiss, Berlin-Banem, Germa ny 1937, Serial No.,180,010

In October 26, 1936 z cum; (01. 250-150) The invention relates tofluorescent screens and .to cathode. ray' tubes containing such screens.It is particularly concemed with screens for making visible pictures ordrawings of any kind, for instance lines, diagrams` and letters. The arrangement according to the invention is especially adapted to producebright television images with changing contents, advertising pictures inblack and white only or oscillograms. 4

Among the obiects 'of the invention are: to produce a fluorescent screen'emitting light under the impact of electrons and in which the emissionof light continues at a uniform rateuntil it is stopped by a reversecontrolling force; to provide a screen structure having light emittingand dark portions adjacent to one another on the same surface, the wholesurface of the screen being bombarded by electrons of the same source;to provide a self controlling fluorescent screen having deflnite areasof two diflerent potentials; to provide a cathode ray tube including afluorescent screen with a material emitting secondary electrons underthe impact of primaryelectrons and a cathode emitting electrons at auniform rate over the whole area' of the screen; to provide a screenstructure on which a fluorescent image is storedby electrlcal chargestorage during the longer part of the period between successivepicturesand cancelled out shortly before a new picture isstored.

Further objects are to provide a method of making a self-controllingscreen and of making a screen structure from which the pictures areeasily obliterated in quick succession.

Other objects of our invention will be apparent as will be speciiicallypointed out in the description forming apart of the speciflcation but`we do not limit ourselves to the embodiments of the invention hereindescribed as various forms may be adopted within the scope of theclaims;

Referring to the drawing:

Flg. 1 is a sectional 'view of a cathode ray cording to the invention. i

Figs. 2 and 3 are dia explaining the operation of the tube representedin Flg. l.

Flg. eis a cross section through a screen structure according to theinvention on an enlarged scale and u Fig. is a plan 'view of a part ofthe screen structure of Flg. 4. j

The tube of Flg. 1 contains within an envelope i a screen 2 ofinsulating material with a layer of fiuorescent material 8. A gridelectrode 4 is arranged in front of the screen and connected. by a lead5 to the outside. The tube contains furthermore a cathode 9 having forinstance the form of a wire. spiral bent to form a ring and connected byconductors and 8 to a heating battery 9. The tube contains furthermore acathode ID of oonventional type for producing a cath- 6' ode ray andconnected to a battery l I. A control electrode |2 is arranged in frontof the cathode !0. This electrode has the form of the wellknown Wehneltcylinder. Externally of the tube j a concentrating coil |3 is situatedaround the 10 neck of the tube Two deflectlng' systems M and I 5 areindicated schematically and constructed in accordance with the usualpractice of oscillographs and television tubes.

In the operation of the device a potential of +500 V. is applied togrid4 with respect to the potential of cathode 6 a's zero-potential. Cathode!0 has a potential of -1000 V. against cathode 6. In order to obt'ain astationary record on the screen the cathode ray emitted by cathode !0,modulated by electrode |2 and concentrated by coii M is deflected overthe screen 2 so that a local potential distribution is produced andstored on the screen in such a manner that certain portions of thescreen have a potential lying above a value of for instance 250 V.against cathode 8 and the remaining portions have a potential below 250V. The potential of 250 V.

corresponds to the critical potential at which electrons hitting thescreenliberate the same number of secondary electrons as primaryelectrons are arriving on the screen.- It is assu'med that thedistribution of potential is' maintained approximately constant for aperiod of time owing to the insulating qualities of the screen 2 85 andthe fluorescent layer 3.

If now the source of electrons 6 is energized the whole screen issprayed diflusely and uniformly with electrons. These electrons areaccelerated to a velocity of 500 V. by the grid 4 40 but they reach thefluorescent layer with a velocity depending upon the potential of therespective surface element of the screen. An element having a potentialabove 250 V. is bombarded by electrons of high velocity 'and more usecondary electrons' are liberated than primary eiectronsreach thiselement.` The potential of the element is made more positive by the lossof. electrons and this increase of potential is con tinued until thepotential of grid I is reached.

` This is the end value which cannot ibe surpassed.

Another elemento! the screen having a potential lower than 250 V. isreached by electrons .having a comparatively low velocity and the numberof secondary electrons is smaller than the u ordinate 1 gives'the ratioof the number of secondary electrons to the number of primary electronsthe abscissa indicates the velocity of the imameting electrons in Volts.Fluorescent materials as ordinarily used in cathode ray tubes forinstance zine sulde have ai characteristic cui-ve Oi reaching the valueof I at 250 V. Ali elements oi the screen having a potential below thiscritical value, show a decrease in potential under the influence of theelectrons emitted by cathode t as indicated by 'potential lying on thedescending part of the characteristic curve in Fig. 2. The criticalpotential is then according to this curve 2700 V. and the positive endpotential is dened by the potential ofgrid ti. v

In order to remove the image stored and appearing on screen E it ispossible to bombard the whole screen by the concentrated ray from sourcei@ so that all parts of the screen take on the high potential and areluminating brightly. In order to control the screen in the reverse'direction and to make -it dark it is possible to lower the potential ofgrid i for a short period of time so that all parts of the screen assumea potential below the critical value. i

The critical value of 250 V. depends upon the material of thefluorescent layer and its treatment and can be changed in fairly widelimits by the correct selection of the material. It is 'also possible tochange the potential on grid 3 and cathode i@ in wide limits.Experiments have shown, that it is possible to maintain luminating anddark portions on the screen adjacent to one another also in case thebase of the screen consists of a metal plate. The insulating propertiesof the fiuorescent material are sufliciently good to build up localcharges on the screen.

In order to obtain a stationary image remaining unchanged it isnecessary to maintain the follow ng Operating conditions explained inconnection with Fig. 3. The fiuorescent material !9 is arranged uponthe'insulating support 8 and bombarded by electrons in the direction ofarrows 20. It is assumed that the fluorescentmaterial has the potentialon the right hand side of the figure as' far as it isrepresented blackand has a potential of 500 V. in the left hand part. The distribution ofthe potential is indicated by curve Zi showing the potential independence from the local extension of the screen. A large difference ofpoten'tials ex sts at the place of arrow 22 and the slope of the localdiflerence in potential depends upon the conductivity of the material.Experiments have shown that the &188398 I lines separating areas of highpotential !rom those of low potential, i. e. separating the bright anddark parts of the screen are subject to a displacement depending uponthe velocity of the electrons emitted by cathode G. In case theelectrons arrive too slowly the dark portions are increaslng and in casethe velocity of the electrons is too high the bright portions of thescreen are increasing. This displacement would destroy the picture aftera certain time and care must be taken to stabilise the picture. rate ofthe' displacement depend also from the material of the screen and thesupport of the screen.

According to the invention the velocity of the diflusely emittedelectrons is exactly adjusted in such a manner that neither adisplacement in one or the other direction occurs. In the abovementioned example of zinc suifide as screen material a voltage of about500 V. has been found as critical for a stationary picture. It'can beassumed that the critical value of the material of corresponds to abouthalf this voltage, i. e. to'

In case the the, screen is' used for producing moving pictures forinstance for cinematographic or television purposes it is necessary toproduce the picture and to remove it within a short period of time offor instance of a sec. able to extinguish or cancel the picture notmomentariiy over the whole' screen surface but only on that part of thescreen on which the new picture is produced immediately thereafter inorder to obtain pictures of uniform brightness which are tree fromfiicker and as bright as possible. A second concentrated cathode ray maybe used for cancelling the pictures; this ray is defiected in a similarmanner as the recording pencil but is displaced a few lines in advanceof the recording pencil. It is also advantageous to divide the screenand/or the grid in front of the screen into a number of sectionsparallel to the lines of the picture. The cancellation is eil'ected inone section only of the screen at a time shortly before the recordingThe direction and' It is prefercathode ray is touching this part of thescreen.

A particularly simple embodiment is represented in Figs. 4 and 5. Fig. 4shows the screen of a tube similar to that of Fig. 1 with two cathodes.The screen is rep'esented on an enlarged scale and consists of a numberof parallel wires 23 insulated from one another. The wires are partlyembedded in the insulating material 24 and are covered with fluorescentmaterial 25. The fluorescent material is covering also the exposedsurface oi' the insulating material 24. Fig. 5 shows a View of thescreen from the side of the cathode. The wires 23 are bent upwardly onthe right hand side and are terminating in or upon the insulatingmaterial forming the carrier, of the screen. A conductor 24 is arrangedat the edge of the screen. A conductor 24 is arranged at the edge of thescren. The strip of material 25 between the conductor 24 and the ends ofwires 23 is semi-conducting so that all the wires 23 are connected byway of this resistance with a source of potential of 500 V. The surfaceof the wires is carefully cleaned between the dotted lines 26 and 21 anda strip of finely divided carbon, for instance graphite, is applied.Furthermore each pair of adjacent wires -is connected by conductingibridges 28.

aieasee 'ment the ray impinges upon the non-emitting strip 25, 2?. Thewires crossed over by the cathode ray at this place receive moreelectrons than they give o so that the potential of these wires issharply lowered. As the strip of graphite is somewhat conducting alsotwo or three adjacent wires will attain a more negative potential. It ispreferable to control the ray during this time for highest intensity andthe defiecting field is controlled preferably in such a manner that theray is remaining for a while upon the strip 26, 2?.

It may be preferable also to make the strip broader than represented inthe drawing. The quick decrease of potential of the wires serves tocancel the illumination in that part of the screen lying near thewires.The screen is therefore brought to darkness a few lines in advance ofthe cathocle ray. The cathode ray is then quickly re.- turned and writesthe second line as indicated by arrow 38. During the return movement ittouches the sections 2 s of wires which are more closely adjacent to itsnext path, so that these wires are made more positive thereby. Duringthe whole time of recording, the wires brought i quickly to a lowerpotential are slowly made more positive by the current flowing throughthe semiconductor 25, so that the wires assume again the function ofgrid 4 of Fig. 1, the lumination near these wires being howevercancelled. Only when the modulated cathode ray is charging definiteparts of the line to` a more positive. potential the illumination startsat these points and will remain under the influence of the electronsemitted by cathodes 6 until the concentrated ray is touching again theupwardly bent edges of the wires between lines 26 and 21.

A screen of this type may contain separate cancelling and acceleratingelectrodes. It is also possible to divide the fiuorescent material intosingle insulated elements by cutting lines into the layer or by sprayingthe fluorescent material through a' grid which is removed afterwards.such a. screen consisting of insulated fiuorescent particles has theadvantage that the displacement efiect described in connection with Fig.3 is minimsed.

As the elements of the screen are luminating either brightly or not atall the pictures are only black and white without half-tones' It ishowever possible to obtain half-tone-pictures by changing the width ofthe luminating lines. The modulated ray m is therefore controlled insuch a manner that chiefly the diameter or size of the spot is changedwhile the intensity of the ray per surface unity is kept approximatelyconstant. The visibility of the lines can be decreased by using aninterlaced scanning method.

It is preferable to use as high a potential as possible between thecathode 6 and the screen. This however is only possible in case thefactor p is reached for a very high velocity of primary electrons.According to the invention a material* is used for the screen in whichthe value is reached above 500 V. or more.

The efiect of the cancelling strip 26, El may be increased .by`multiplying the effect of decreasing the potential by a multistagesecondary emisslon multiplier for instance of the grid type.

The potential distribution maintained on the screen can also be used forcontrolling 'other effects. It is also within the scope of the inventionto produce an electron optical image of the screen surface upon anothersurface and to sepa- 'rate locally the production-of the potentialdistribution and the luminous efifect.

We claim: i

1. In a cathode ray tube an insulating plate, a fluorescent screen onsaid plate, a source of electrons for uniiormly bombarding the wholearea of said screen with'electrons, an accelerating electrode in frontof said screen having a fixed potential above the critical value atwhich the number of secondary electrons emitted by the screen exceedsthe number of primary electrons,

means for charging elemental areas of said screen to differentpotentials, and means for intermittently bringing the acceleratingelectrode to negative potential.

2. In a cathode ray tube a plate, fluorescent insulated elemental areason said plate, a source of electrons for uniformly bombardng the wholearea of said plate with electrons, an accelerating electrode having afixed potential of twice the critical value at which the number ofsecondary electrons emitted by the plate exceeds the number of primaryelectrons, and means' for charging elementa areas of said plate todiflerent potenv tials.

mmnncn RING. anoaa worse.

